Impressive clinical, epidemiological, and experimental evidence shows that negatively reinforcing (aversive or "stressful") conditions can contribute to a variety of physical and mental illnesses including hypertension and cardiovascular damage. Considerable clinical and epidemiological evidence suggests that positively reinforcing conditions, such as social support, can counteract the negatively reinforcing ones, but there have been virtually no experimental studies of such positive effects except for some behavioral studies of counterconditioning in which physiological, cardiovascular, and hormonal effects have not been systematically studied with modern techniques and in most of which adequate control groups have not been used. We shall investigate how strong an aversive electric shock can have its effects neutralized by counterconditioning (making it the signal for food for a hungry animal) without producing adverse effects on heart rate, blood pressure, plasma cortisosterone, urine formation and osmolarity, and behavioral measures of distress. Will pain-counteracting effects occur and, if so, to what extent will they occur in the different systems to be measured? Which pain-suppressing effects, if any, may be counteracted by an opiate antagonist? Over a period of several weeks, very hungry rats will receive 20 trials during ond hour per day of gradually increasing shocks via fixed electrodes on their tails. In the experimental group, each shock will signal the delivery of a highly preferred food. In different control groups, which will be of some interest in their own right, shock and food will occur in random sequence, some animals will receive shock only, food only, or only confinement in the apparatus, with an added post-test of sudden exposure to full-strength shock in the last of these two groups. Blood pressure will be measured via a chronic catheter into the aorta, heart rate via electrodes in the thoracic cavity, rates of urine formation and osmolarity and glucosuria via a chronic catheter into the bladder, and plasma corticosterone by analysis of blood secured via the chronic catheter and/or by decapitation at terminal sacrifice. Behavioral measures will be recordings of struggling, squealing, failure to eat (CER), and defecation in the experimental apparatus.